In order of -deadline scheduling to be effective and useful, it isimportant that some method of having the allocation of the availableCPU bandwidth to tasks and task groups under control.This is usually called "admission control" and if it is not performedat all, no guarantee can be given on the actual scheduling of the-deadline tasks.

Since when RT-throttling has been introduced each task group have abandwidth associated to itself, calculated as a certain amount ofruntime over a period. Moreover, to make it possible to manipulatesuch bandwidth, readable/writable controls have been added to bothprocfs (for system wide settings) and cgroupfs (for per-groupsettings).Therefore, the same interface is being used for controlling thebandwidth distrubution to -deadline tasks and task groups, i.e.,new controls but with similar names, equivalent meaning and withthe same usage paradigm are added.

However, more discussion is needed in order to figure out howwe want to manage SCHED_DEADLINE bandwidth at the task group level.Therefore, this patch adds a less sophisticated, but actuallyvery sensible, mechanism to ensure that a certain utilizationcap is not overcome per each root_domain (the single rq for !SMPconfigurations).

Another main difference between deadline bandwidth management andRT-throttling is that -deadline tasks have bandwidth on their own(while -rt ones doesn't!), and thus we don't need an higher levelthrottling mechanism to enforce the desired bandwidth.

This patch, therefore: - adds system wide deadline bandwidth management by means of: * /proc/sys/kernel/sched_dl_runtime_us, * /proc/sys/kernel/sched_dl_period_us, that determine (i.e., runtime / period) the total bandwidth available on each CPU of each root_domain for -deadline tasks; - couples the RT and deadline bandwidth management, i.e., enforces that the sum of how much bandwidth is being devoted to -rt -deadline tasks to stay below 100%.

This means that, for a root_domain comprising M CPUs, -deadline taskscan be created until the sum of their bandwidths stay below:

M * (sched_dl_runtime_us / sched_dl_period_us)

It is also possible to disable this bandwidth management logic, andbe thus free of oversubscribing the system up to any arbitrary level.